Typically, longitudinal media, such as magnetic tape or optical tape, is employed as a secondary storage media for storing large amounts of data which is accessed sequentially or infrequently (such as archival). To accommodate the large amounts of data, the tracks are placed closely together and a track following servo system is employed to allow the tape head to accurately follow the data tracks.
An example of a track following servo system particularly adapted to tape comprises a timing based servo pattern described in coassigned U.S. Pat. No. 5,689,384. The servo patterns are comprised of magnetic flux transitions recorded in continuous lengths at non-parallel angles, such that the timing between the servo transitions read from the servo pattern at any point on the pattern varies continuously as the head is moved across the width of the servo pattern. Thus, the relative timing of transitions read by a servo read head varies linearly depending on the lateral position of the head. Speed invariance is provided by utilizing a group of interlaced pairs of transitions and determining the ratio of two timing intervals, the interval between two like transitions ratioed with the interval between two dissimilar transitions. Synchronization of the decoder to the servo pattern may be accomplished by having two separate groups of pairs of transitions, each group having a different number of pairs of transitions. Thus, the position in the set of groups is readily determined by knowing the number of pairs of transitions in the present group.
In addition, the determination of the longitudinal position of the tape is important. Often, the data is transferred to the longitudinal media for writing on the media by streaming. Similarly, the data is often read on a continuous basis for much of the data. The data transfer is, however, often subject to interruption while the media continues to move at its continuous nominal velocity. Thus, the media must be stopped, and later restarted. On restart, the media position must be correlated and resynchronized with respect to the data set sequence.
Hence, as described in the incorporated Albrecht et al. patent, in addition to the determination of the lateral position of a head with respect to the width of a tape by means of the timing based servo pattern, data may be modulated into, or superimposed on, the servo pattern to allow determination of the longitudinal position of a tape. In one example, a pair of transitions are each oppositely facing slanted lines formed into groups, or "frames", of multiple pairs of transitions, the similarly sloped transitions of each pair arranged together in a burst, with the bursts and the frames each separated by servo gaps. In one embodiment, at least one transition of the repeated pairs in a frame is shifted longitudinally with respect to other of the transitions, the shifted transitions comprising the modulated data information.
Data information of the type proposed by the incorporated Albrecht et al. patent is employed to provide prerecorded longitudinal position registration data information in the timing based servo patterns of the longitudinal servo track. As an example, the servo information comprises alternating 5 and 4 servo frames wherein each bit of the longitudinal position information is encoded into a pair of the bursts of a 5 servo frame by shifting the longitudinal position of the stripes of the pair in opposite directions, shifts of the pair in first directions representing a "1", and shifts in the second, opposite, directions representing a "0". The absolute location along the length of tape is represented by a "Longitudinal POSition", or "LPOS", word composed of 36 bits from 36 frames, consisting of 24 bits forming a position word, a 4-bit symbol (such as manufacturer's or user's data) and an 8-bit synchronization mark. The bits are typically detected by detecting the shifts of the pairs in the first or the second directions by measuring the intervals between each shifted transition stripe and the adjacent unshifted transition stripes.
Media debris and defects can cause extra flux transitions to be detected or inhibit detection of written flux transitions. Measurement of the intervals for each shifted transition stripe is not possible in the presence of these errors. But, the modulated longitudinal position data is required on a constant basis to provide accurate longitudinal positioning of the tape and therefore must be reliably detected. In normal data recording, extensive error correction codes are provided which allow detection and correction of errors. However, no error correction bytes are allowed under the current standard for LPOS words. Doing so would increase the length of the LPOS words, possibly unacceptably.